As is widely known, a centrifugal compressor functions to pass a gas in a radial direction of a rotating impeller and to compress a fluid such as the gas using a centrifugal force generated at that time. As such a centrifugal compressor, a multistage centrifugal compressor which includes impellers in multiple stages in an axial direction thereof and compresses a gas stepwise is known (see Patent Literature 1). The multistage centrifugal compressor will be described in brief with reference to an accompanying drawing.
As shown in FIG. 6, a compressor 101 includes a casing 5 in which an inlet and an outlet not shown are formed, a rotation shaft 2 that is rotatably supported by the casing 5 with a bearing section (not shown) interposed therebetween, a plurality of impellers 3 that are attached at predetermined intervals along the axial direction of the rotation shaft 2, and a flow channel 4 that connects the impellers 3 to cause a gas which is compressed stepwise to flow. The casing 5 includes a shroud casing 5a and a hub casing 5b. 
Each impeller 3 mainly includes a disc-like hub 13 of which the diameter is gradually enlarged to one side (rear stage side) in the axial direction, a plurality of vanes 14 that are radially attached to the hub 13, and a shroud 15 that is attached to cover the tip sides of the plurality of vanes 14 in the circumferential direction.
The flow channel 4 includes a compression flow channel 17 and a return flow channel 118. The compression flow channel 17 is a flow channel which is defined by a vane attachment surface of the hub 13 and an inner wall surface of the shroud 15 facing the vane attachment surface. The return flow channel 118 includes a suction section 119, a diffuser section 120, and a return bend section 121.
The suction section 119 includes a straight channel 122 through which a gas flows from the outside in the radial direction to the inside in the radial direction and a curved corner channel 123 that converts the flow direction of a fluid flowing from the straight channel 122 into the axial direction of the rotation shaft 2 and guides the fluid to the impeller 3. The diffuser section 120 is a channel extending to the outside in the radial direction and causes a fluid compressed by the impeller 3 to flow to the outside in the radial direction. The return bend section 121 is a curved channel that converts the flow direction of the fluid passing through the diffuser section 120 into the inside in the radial direction and sends the fluid out to the suction section 119.
Accordingly, a fluid G sequentially flows through the first-stage suction section 119, the compression flow channel 17, the diffuser section 120, and the return bend section 121 and then sequentially flows through the second-stage suction section 119, the compression flow channel 17, . . . , whereby the fluid is compressed stepwise. The straight channel 122 of the suction section 119 is provided with a plurality of return vanes 125 that are radially arranged and that partition the straight channel 122 in the circumferential direction. The plurality of return vanes 125 are arranged over the entire width of the straight channel 122.